Second-degree AV block
An intermittent failure of AV conduction in which some P waves are not followed by a QRS.
Key ECG features
- ✓Some P waves not followed by a QRS
- ✓P waves march regularly straight through the drop
- •R-R containing the dropped beat is longer
- ±Mobitz I: PR lengthens before the drop
- ±Mobitz II: PR fixed, drop abrupt
✓ required · • typical · ± may be present
ECG characteristics
The P waves continue at their own regular rate and march straight through the dropped beat, while the R-R interval containing it is visibly longer. It divides into Mobitz type I (Wenckebach), with progressive PR prolongation before the drop, and Mobitz type II, with a fixed PR and abruptly non-conducted P waves.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Mobitz I is commonly vagally mediated; Mobitz II more often reflects structural disease of the conduction system below the AV node. Drugs that slow AV conduction and electrolyte disturbance can produce either.
Why it matters
Telling the two apart matters, since Mobitz II and high-grade block are more likely to progress to complete block and to prompt pacing evaluation.
Differentiate from
- Second-degree AV block (Mobitz I)Compare the PR interval immediately before the dropped beat with the one immediately after: progressive lengthening means Mobitz I.
- Second-degree AV block (Mobitz II)A PR interval that stays fixed right up to the dropped beat means Mobitz II, which carries the worse prognosis.
- Sinus pause & arrestA dropped beat leaves a P wave with no QRS after it; a sinus pause has no P wave either.
- Third-degree (complete) AV blockSome P waves still conduct here, with a consistent PR interval. In complete block none conduct at all.
- First-degree AV blockIf every P wave still conducts and nothing is dropped, the PR interval is merely long - that is first-degree block, not second.